Leeanne,

In general principle warm air has the capacity to retain more moisture
in the form of vapor, hence higher relative humidity in the summer
than in the winter. Such a dramatic drop in both temperature and
relative humidity suggests that perhaps a window or door was left open
that ususally is not - or in some modern HVAC systems a vent to the
outside air can do a similar thing. Double-check the chart and timing
of this event and try to coorelate it with any other possible events -
check it with your building engineer against any HVAC records, and
with security, etc. Other questions to look at are whether this room
has exterior walls, alot of windows, exterior doors, etc.If there were
big changes in the ourdoor weather sometimes those efffects can
register in spaces that are most exposed. Is this room more
susceptible to fluctuation than other spaces?

When I encountered a smiliar phenomenon when I worked as a conservator
at a large history museum (in my instance both temperature and
relative humidity went dramatically up) I was able to compare
information on my charts with the engineer and actually found a flaw
in their system, because of the placement of room hygrothermographs we
had a more accurate reading than did the HVAC sensors that were in the
ducts.

Cheers!
Dave

David Harvey
Conservator
Los Angeles, California, USA

On 10/21/05, Westwood Leeanne <[log in to unmask]> wrote:
>
>
> Hi to everyone
>
> Please help!! I am just analyzing the data from our museums electronic
> relative humidity dataloggers and I have found an unusual fluctuation in the
> readings. At one point over the weekend, when we are closed, the temperature
> in a certain room consistently dropped, but at the same time, so did the
> Relative Humidity!. Please can anyone explain what would cause the relative
> humidity to drop at the same time as the temperature?
>
> Thanks
>
> Leeanne Westwood
>
> Senior Museum Assistant
>
> Valence House Museum
>
> London Borough of Barking and Dagenham
>
> Becontree Avenue
>
> Dagenham
>
> RM8 3HT
>
> Tel: 020 8270 6744
>
> Fax: 020 8270 6868
>
> [log in to unmask]
>
> Website: www.barking-dagenham.gov.uk
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